A bridge must be constructed not only to sustain its own weight, including the floor, commonly referred to as "dead load," but also the weight and impact of traffic, referred to as the "live load." It is important to the bridge designer that the dead load be reduced as much as possible without impairing its capacity to safely carry the live load. Much study has been given to the construction of bridge floors since they contribute to such a large part of the entire dead load of the bridge, and also because they are directly subject to the wear and deteriorating impact of traffic. In addition, the floors are exposed to the elements and the corrosive effect of air-borne pollution. Also, in large areas of the nation they are exposed to the destructive effect of snow-melting and de-icing chemicals.
When a bridge floor must be replaced, not only is it costly, but it will usually seriously disrupt the flow of traffic across it for extended periods of time.
Open grid floors for bridges have the lowest weight per square foot of flooring heretofore used. They have welded joints at the intersection of the cross bars and bearer bars, and in many cases apparently sound welds are in fact imperfect with the result that after a period of time under severe traffic conditions bars not infrequently become increasingly loose, noisy, and eventually the grating must be replaced. This is especially true where the welding usually imparts a bow to a grating panel which is subsequently flattened by rolling, tending to excessively stress the welds. While they are in service, open grid floors do have the advantage that wind pressures above and below the deck are equalized, which is especially desirable on long bridge spans where unequal wind pressures above and below the bridge floor produce destructive forces. Whether filled with concrete, or used as an open grid, such grid structures have nevertheless heretofore provided one of the best available floors.